A seat frame having a seat cushion frame supporting a vehicle seat and a seatback frame supporting a seatback of the vehicle seat, connected to the seat cushion frame and equipped with a retractor (a seat belt retaining member), is known. The seat cushion frame includes a front-rear direction frame extending along a front-rear direction of a seat cushion. The seatback frame includes an upward-downward direction frame extending along an upward-downward direction of a seatback.
A seat frame with a recliner connected to a rear end portion of a main side cushion side frame (the front-rear direction frame) and the lower end portion of a main side back side frame (the upward-downward direction frame) is disclosed in JP2002-59770A. According to the seat frame disclosed in JP2002-59770A, the recliner includes a lower arm and an upper arm each in a flat plate form. The lower arm is connected to the main side cushion side frame. The upper arm is connected to the main side back side frame. The main side cushion side frame and the main side back side frame both have closed cross sectional structures. The lower arm is inserted into the main side cushion side frame from the upper opening, biased to a side wall of the main side cushion side frame by collars, and fastened by bolts to be retained in the biased state. The upper arm is inserted into the main side back side frame from the lower opening, biased to a side wall of the main side back side frame by collars, and fastened by bolts to be retained in the biased state.
According to the seat frame disclosed in JP2002-59770A, a side frame with closed cross sectional structure and the recliner are fastened by bolts. In this construction, when a tensile force is applied from a seat belt to the main side back side frame, rigidity of the side frame in cross section changes significantly depending on a direction of the applied tensile force. Also in this construction, a possibility for a buckling distortion to occur is high because the stress by the tensile force from the seat belt concentrates on the bolted portions.
A need thus exists for a seat frame, which is not susceptible to the drawback mentioned above.